The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

Abstract. The Great Oxidation Event (GOE) was a 200 Myr transition circa 2.4 billion years ago that converted the Earth's anoxic atmosphere to one where molecular oxygen (O₂) was abundant. This rise in O₂ is thought to have substantially throttled hydrogen (H) escape and the associated water (H₂O) loss. Since the GOE, the amount of hydrogen escaping has been influenced by the methane (CH₄) mixing ratio and the diffusion of H₂O into the upper atmosphere. In this study we use WACCM6, a three-dimensional Earth System Model to simulate Earth's atmosphere and predict the diffusion-limited escape rate of hydrogen due to varying O₂ concentrations based on atmospheric estimations from the GOE onward, ranging between 0.1 % PAL to 150 % PAL, where PAL is the present atmospheric level of 21 % by volume. O₂ indirectly acts as a control valve on the amount of hydrogen atoms reaching the homopause in the simulations: less O₂ leads to decreased O₃ densities, reducing local temperatures by up to 5 K, which increases H₂O freeze-drying. For the considered scenarios, the maximum difference in the total H mixing ratio at the homopause and calculated diffusion-limited escape rates is a factor of 3.2 and 4.7, respectively, with the prescribed CH₄ mixing ratio setting a minimum diffusion escape rate of ≈2 × 10¹⁰ mol H yr^-1. These numerical predictions support geological evidence that the majority of Earth's hydrogen escape occurred prior to the GOE. Our work demonstrates that estimations of how the tropical tropopause layer and the associated hydrogen escape rate evolved through Earth's history requires 3D chemistry-climate models which include a global treatment of water vapour microphysics.